This invention relates to a method for making a nanocrystal-based material capable of emitting light at a sufficiently wide range of wavelengths to appear white, making it suitable as a phosphor for visible illumination. Lamp phosphors for visible illumination are typically designed to be strongly absorbing at the energy corresponding to the Hg discharge lines of current fluorescence light tubes, around 254 nm. The “wall-plug” efficiency of fluorescent lighting is very good, about 28%. However, the desire to eliminate the toxic Hg in such sources and replace it with materials such as the inert gas Xe requires the development of new phosphors with longer wavelength absorbance, ˜400 nm. Additionally, the color rendering of current lighting could be improved as the dominance of less efficient incandescent lighting over fluorescent lighting in residential applications demonstrates.
Alternative excitation sources, such as near-UV solid state GaN LEDs, also require new phosphors with strong absorbance near 400 nm. Unfortunately, phosphors that emit broadly in the visible range of 450-650 nm with little or no self-absorbance or scattering do not exist. Since either scattering or self-absorption in a conventional phosphor leads to loss of light extraction and overall efficiency in a lighting device, new types of phosphors are needed
Phosphors based upon semiconductor nanocrystals, often termed nanophosphors, have certain desirable properties for both lamp and LED applications. In the latter application, in particular, the ability to determine the absorption characteristics by both nanocrystal size and material type should allow one to make a material with a large absorption in the 380-420 nm regime, optical transparency in the visible regime, and negligible scattering in the visible region.
All semiconductor nanocrystals made simply by high-temperature decomposition of organometallic precursors result in phosphors with strong overlap between absorbance and photoluminescence, (PL), as well as narrow-linewidth PL emission. These two characteristics mean that blending of various sizes of nanocrystals is required to achieve a broad, white emission. The resulting blend will not be optically transparent to visible light. Thus, the smallest nanocrystals with the shortest emission wavelength need to be closest to the excitation source so that their PL can be used to excite the other larger nanocrystals emitting at longer wavelengths. Each absorption/emission event lowers the overall efficiency. The nanophosphor layering must also occur at large optical densities to ensure that all the exciting light is captured in the short distances of 1-3 mm available in most LED geometries.
To eliminate the need to mix and layer different size nanocrystals to produce white light, an ideal nanophosphor should have independently adjustable absorbance and emission energies. This is achieved in conventional lamp phosphors by the choice of the absorbing semiconductor matrix material and suitable luminescent ions (termed dopants, activators, or luminescent centers). If a nanocrystal is sufficiently small that carrier recombination occurs almost completely from surface states or interface states, a similar decoupling of absorption energy from emission energy is possible. For example, in nanocrystalline Si, the photogenerated carriers in small, 1-3 nm clusters, have been calculated to rapidly diffuse to the surface where they are believed to be trapped in a wide energy range of “sub-gap” interface states from which recombination and light emission may occur (Zhou et al., Nano Letters 3 (2003) p. 163-167.)
A number of patents exist concerning light-emitting nanocrystals.
Gray et al. (U.S. Pat. No. 5,985,173) concerns phosphors having a high light output level, no or few surface defects, and exhibiting minimal non-radiative recombination. These objects are accomplished by surrounding a doped host with a shell having a band gap either larger than the bandgap of the doped host or having no states within 20 meV to 200 meV of said band edges, or having a bandgap offset from said bandgap of the doped host such that an electron or hole from the doped host material is reflected back into the doped host material.
Gray et al. (U.S. Pat. No. 6,090,200) concerns the method for making the phosphors claimed in U.S. Pat. No. 5,985,173.
Gray et al. (U.S. Pat. No. 6,379,583) concerns nanocrystalline phosphors comprising a semiconductor host compound doped with one or more of several dopant atoms wherein said doped nanocrystalline phosphor has an average of about one or less dopant ions per nanocrystalline phosphor particle.
Gallagher and Bhargava (U.S. Pat. No. 6,048,616) concerns doped encapsulated semiconductor nanoparticles of a size (<100 Angstroms) which exhibit quantum confinement effects. The nanoparticles are precipitated and coated with a surfactant by precipitation in an organometallic reaction. The luminescence of the particles may be increased by a further UV curing step.
Bhargava (U.S. Pat. No. 5,455,489) concerns displays comprising doped nanocrystal phosphors. The phosphor material used in the displays comprises doped nanocrystals: tiny, separated particles of the order of 100 Angstroms or less and thus exhibiting quantum confinement properties. These quantum-confined particles of certain luminescent materials when doped with an activator yield ultra-fast and efficient phosphors.
Bhargava and Gallagher (U.S. Pat. No. 6,241,819) concerns a method of making doped semiconductor nanocrystals. The method involves first making a polymer matrix containing dopant and one component of the host material, drying the matrix, immersing polymer matrix in second solution, diffusing in second component to react and grow doped nanocrystals within the polymer matrix, removing the polymer matrix from the second solvent, and drying the matrix.
Ihara et al. (U.S. Pat. No. 6,447,696) reports a manufacturing method for a nanocrystal light emission substance having a nanostructure crystal, doped with an activator and cured with ultraviolet light. The nanocrystal light emission substance is synthesized by a liquid phase co-precipitation process. During the liquid phase reaction, an organic acid, such as acrylic acid or methacrylic acid, is added. Alternatively, a high molecular organic acid, such as polyacrylic or polymethacrylic acid, polystyrene, is added after the liquid phase reaction. The resulting substance is then cured with ultraviolet light.
The preceding patents incorporate dopants, also called activators or luminescent centers, within the nanocrystal to achieve light emission at a wavelength determined by the electronic properties of the dopant in the nanocrystal.
Lawandy (U.S. Pat. No. 5,882,779) reports a display screen comprising a class of high efficiency (e.g. >20%) materials for use as display pixels. The materials are comprised of nanocrystals such as CdSSe, CuCl, GaN, CdTeS, ZnTe, ZnSe, ZnS, or porous Si or Ge alloys which may or may not contain a luminescent center. The nanocrystals may be doped with a luminescent center such as Mn2+ or a transition metal. The nanocrystals have passivated surfaces to provide high quantum efficiency. The nanocrystals have all dimensions comparable to the exciton radius (e.g., a size in the range of approximately 1 nm to approximately 10 nm). A quantum dot nanocrystal display phosphor has a size selected for shifting an emission wavelength of a constituent semiconductor material from a characteristic wavelength observed in the bulk to a different wavelength.
Lakowicz et al. (U.S. Pat. No. 6,660,379) reports CdS nanoparticles formed in the presence of an amine-terminated dendrimer that show blue emission and the method for making these nanoparticles. The emission wavelength of these nanoparticles depends on the excitation wavelength. The CdS/dendrimer nanoparticles display polarized emission with the anisotropy rising progressively from 340 to 420 nm excitation, reaching a maximal anisotropy value in excess of 0.3. Polyphosphate-stabilized CdS nanoparticles are described that display a longer wavelength red emission maximum than bulk CdS and display a zero anisotropy for all excitation wavelengths.